Exhaust nozzle including idle thrust spoiling

ABSTRACT

A method and apparatus are disclosed for spoiling thrust from combustion gases discharged from an aircraft gas turbine engine at ground idle operating condition. The method comprises the step of positioning a secondary exhaust flap of the exhaust nozzle to form a diffuser for the combustion gases at the ground idle operating condition for maintaining attachment of the combustion gases along the secondary exhaust flaps for spoiling thrust. An exemplary and preferred exhaust nozzle is provided wherein the secondary exhaust flaps are positionable in part independently of primary exhaust flaps so that the primary and secondary flaps may be disposed together in different positions during ground idle, dry, and augmented operating conditions of the engine. Thrust spoiling allows the engine to be operated at relatively high core speeds for providing bleed-air at relatively high pressure or electrical power from a generator, or both, without attendant relatively high thrust from the engine at the ground idle operating condition.

This is a division of application Ser. No. 07/479,503, filed Feb. 12,1990 now U.S. Pat. No. 5,140,809.

TECHNICAL FIELD

The present invention relates generally to aircraft gas turbine enginevariable exhaust nozzles, and, more specifically, to a means and methodfor spoiling ground idle thrust from such engines.

BACKGROUND ART

Conventional military fighter aircraft are powered by high performancegas turbine engines having relatively high thrust-to-weight ratio forproviding high acceleration rates of the aircraft. The aircraft gasturbine engine typically includes a variable area converging-divergingexhaust nozzle at a downstream end of a conventional afterburner oraugmenter. The exhaust nozzle includes primary and secondary exhaustflaps which define converging and diverging channels through whichcombustion gases from the engine are discharged for generating thrust.

The exhaust nozzle is conventionally positionable for generally twomodes of operation: a dry engine operating condition, wherein theafterburner is deactivated, and the primary and secondary exhaust flapsare in a generally fully closed position; and a wet, or augmentedoperating condition wherein the afterburner is activated and burnsadditional fuel for providing increased thrust, and the primary andsecondary flaps are in a generally fully open position. Of course, theexhaust nozzle primary and secondary flaps are also conventionallypositionable at intermediate positions in each of the dry and wet modes.

A conventional military aircraft may also include an EnvironmentalControl System (ECS) which requires extraction of engine compressorbleed-air at pressures typically at least 40 psia. Furthermore, theengine typically includes a generator requiring a minimum shaft rpm forproviding electrical output power for the aircraft.

With the aircraft operating in take-off and cruise modes of operationand during dry and wet modes of operation, the engine is amply effectivefor providing the required ECS bleed-air as well as electrical powerfrom the generator. Furthermore, the engine is operable in aconventional ground idle operating condition wherein the throttle is setback to a minimum thrust and power setting of the engine, which istypically less than about 6% maximum dry thrust of the engine. However,in order to obtain acceptable levels of ECS bleed-air and acceptablepower from the generator, the ground idle operating condition requires acore engine speed typically of about 70% of maximum speed, although theconventional fan speed is substantially lower.

Since the engine is a high performance engine having a highthrust-to-weight ratio, this relatively high core speed results insubstantial thrust from the engine during the ground idle operatingcondition. This thrust is typically sufficient for causing the aircraftto roll on the ground unless braking is utilized. Of course, suchbraking during ground idle operating condition, substantially increaseswearing of the aircraft's brakes, tires and wheels. Furthermore, duringicy runway and taxiway conditions, braking through the wheels isrelatively ineffective for accommodating the ground idle operatingcondition thrust.

Yet further, these aircraft are typically operated world wide andoperate on a wide variety of runways/taxiway surface conditions,including water and ice accumulation, and with varying degrees of rampcongestion of other aircraft. Under these conditions, a relatively lowlevel of ground idle thrust is desirable for maintaining safe landingand taxiing speeds.

Accordingly, the aircraft's brakes, as above described, may be utilizedfor accommodating the relatively high ground idle thrust encounteredduring landing, taxiing, and standing, but this is generally undesirablein view of the increased wear associated therewith. Of course, theground idle operating condition of the engine could be preselected forobtaining relatively low core engine speeds for reducing ground idlethrust from the engine. However, if the core engine speed is so reduced,acceptable ECS bleed-air and generator output will not be obtained fromthe engine, thus requiring an auxiliary compressor and generator. Thisis undesirable in view of the increased weight, cost and complexity ofsuch systems in the aircraft.

OBJECTS OF THE INVENTION

Accordingly, one object of the present invention is to provide a methodand apparatus for spoiling ground idle thrust from an aircraft gasturbine engine.

Another object of the present invention is to provide a new and improvedvariable area gas turbine engine exhaust nozzle.

Another object of the present engine is to provide a gas turbine engineexhaust nozzle having primary and secondary exhaust flaps positionablefor spoiling ground idle thrust from the engine.

Another object of the present invention is to provide an aircraft gasturbine engine exhaust nozzle effective for providing acceptable levelsof ECS bleed-air at ground idle operating condition while spoilingground idle thrust.

Another object of the present invention is to provide an aircraft gasturbine engine exhaust nozzle operable in wet and dry modes, andoperable in a ground idle operating condition wherein ground idle thrustfrom the engine requires reduced amounts of wheel braking for preventingan aircraft from rolling on its wheels.

DISCLOSURE OF INVENTION

The invention includes a method and apparatus for spoiling ground idlethrust from an aircraft gas turbine engine. The apparatus includes avariable area gas turbine engine exhaust nozzle having primary andsecondary exhaust flaps which are positionable differently during groundidle, dry, and augmented operating conditions of the engine. Theapparatus is effective for practicing the method of positioning asecondary exhaust flap to form a diffuser for spoiling thrust fromcombustion gases at the ground idle operating condition and is effectivefor maintaining attachment of the combustion gases along the secondaryexhaust flaps.

BRIEF DESCRIPTION OF DRAWINGS

The novel features believed characteristic of the invention are setforth and differentiated in the claims. The invention, in accordancewith a preferred, exemplary embodiment, together with further objectsand advantages thereof, is more particularly described in the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 is a schematic representation of an aircraft having two turbofangas turbine engines including exhaust nozzles in accordance with oneembodiment of the present invention.

FIG. 2 is a schematic representation of one of the engines, including anexhaust nozzle, powering the aircraft illustrated in FIG. 1.

FIG. 3 is an end view of the engine illustrated in FIG. 2 taken alongline 3--3 showing an upstream end view of the exhaust nozzle thereof.

FIG. 4 is a schematic representation of the positions of the primary andsecondary exhaust flaps of the exhaust nozzle of the engine illustratedin FIG. 2 during a ground idle operating condition in accordance with anexemplary embodiment of the present invention.

FIG. 5 is a schematic representation of the positions of the primary andsecondary exhaust flaps of the exhaust nozzle of the engine illustratedin FIG. 2 during a dry operating condition of the engine.

FIG. 6 is a schematic representation of the positions of the primary andsecondary exhaust flaps of the exhaust nozzle of the engine illustratedin FIG. 2 during an intermediate, augmented operating condition of theengine.

FIG. 7 is a schematic representation of the positions of the primary andsecondary exhaust flaps of the exhaust nozzle of the engine illustratedin FIG. 2 during a maximum augmented operating condition of the engine.

FIG. 8 is a partly schematic, transverse sectional view of the upperhalf of the exhaust nozzle illustrated in FIG. 2.

FIG. 9 is a partly schematic, plan view of portions of the exhaustnozzle illustrated in FIG. 8 taken along line 9--9.

MODE(S) FOR CARRYING OUT THE INVENTION

Illustrated in FIG. 1 is a schematic representation of a highperformance, military fighter aircraft 10 including two turbofan gasturbine engines 12 in accordance with a preferred, exemplary embodimentof the present invention. The aircraft 10 includes a plurality ofconventional, retractable, wheels 14 and conventional brakes 16operatively connected thereto. The engines 12 are effective forgenerating combustion gases 18 which are effective for providing thrustto power the aircraft 10. In FIG. 1, the aircraft 10 is shown as taxiingon a runway 20 under the power of the combustion gases 18.

Illustrated in FIG. 2 is one of the identical turbofan engines 12illustrated in FIG. 1, shown schematically. The engine 12 includes anannular casing 22 disposed coaxially about a longitudinal centerlineaxis 24. The engine 12 further includes a conventional inlet 26 forreceiving ambient air 28 and channeling the air 28 through aconventional fan 30. A conventional core engine 32 is disposedimmediately downstream of the fan 30 and includes in serial flowcommunication, a conventional compressor 34, combustor 36, and highpressure turbine (HPT) 38. The HPT 38 powers the compressor 34 through afirst shaft 40 fixedly connected therebetween.

The engine 12 further includes a conventional low pressure turbine (LPT)42 disposed downstream from the HPT 38 and in flow communicationtherewith, for powering the fan 30 through a second shaft 44 extendingtherebetween. Conventionally operatively connected to the first shaft 40is a conventional generator 46 for providing electrical power to theengine 12 and the aircraft 10. Conventionally operatively connected tothe compressor 34 is a conventional Environmental Control System (ECS)48 which receives bleed-air 50 from the compressor 34.

The engine 12 further includes a conventional afterburner, or augmenter52 disposed downstream of the LPT 42. The afterburner 52 includes anannular casing, or tail pipe, 54 extending conventionally downstreamfrom the casing 22. The afterburner 52 includes a conventionalcombustion liner 56 which confines the combustion gases 18.

The combustion gases 18 are formed from a portion of the inlet airflow28 which is channeled through the core engine 32 wherein it is mixedwith fuel and ignited in the combustor 36 and discharged through the HPT38 and the LPT 42. Another portion of the inlet airflow 28 bypasses thecore engine 32 to the afterburner 52 for cooling the liner 56. A portionof that airflow 28 is conventionally channeled radially inward of theliner 56. During a dry operating condition or mode of the engine 12, theafterburner 52 is deactivated and the combustion gases 18 dischargedfrom the LPT 42 are passed through the afterburner 52 without any fueladdition. However, during a wet, or augmented operating condition ormode of the engine 12, additional fuel is conventionally added to thecombustion gases 18 discharged from the LPT 42 and the portion of theairflow 28 bypassing the core engine 32 and channeled radially inwardsof the liner 56, and conventionally ignited in the afterburner 52 forproviding the combustion gases 18 with additional energy and velocity,and thereby thrust, for powering the engine 12 and the aircraft 10.

The engine 12 further includes an exhaust nozzle 58 in accordance with apreferred, exemplary embodiment of the present invention disposed at adownstream end 60 of the afterburner 52. In an exemplary embodiment, theexhaust nozzle 58 is axisymmetric about the longitudinal centerline axis24 and includes a plurality of circumferentially spaced conventionalprimary exhaust flaps 62 and a plurality of circumferentially spacedconventional secondary exhaust flaps 64 extending downstream therefrom.A plurality of circumferentially spaced conventional fairings 66 jointhe secondary flaps 64 to the casing 54.

FIG. 3 illustrates an upstream looking view of the exhaust nozzle 58showing the axisymmetric arrangement of the fairings 66 behind which arehidden the primary and secondary flaps 62 and 64.

The engine 12 illustrated in FIG. 2 is operable at a ground idleoperating condition or mode of minimum output thrust from the gases 18discharged therefrom. The ground idle mode is preselected for obtaininga predetermined pressure of the bleed-air 50 acceptable for operatingthe aircraft Environmental Control System 48, which in the exemplaryembodiment illustrated in a pressure of at least 40 psia. Furthermore,the ground idle mode is also selected for powering the generator 46 at asuitable rpm for providing an acceptable level of electrical outputpower therefrom. In the exemplary embodiment illustrated, the coreengine first shaft 40 is operated at about 70% maximum speed thereofduring the ground idle mode. The speed of the second shaft 44 and thefan 30 is substantially lower than the speed of the first shaft 40during the ground idle mode as conventionally obtained.

Utilizing such a relatively high speed of the core engine first shaft 40for the ECS 48 or the generator 46, or both, would result in undesirablyhigh thrust from the combustion discharge gases 18 if a conventionalexhaust nozzle were utilized. This would occur since a conventionalvariable area exhaust nozzle is typically configured for only two modesof operation: a dry mode of operation wherein the primary and secondaryexhaust flaps thereof are generally closed, and an augmented mode ofoperation where the primary and secondary exhaust flaps thereof are in agenerally open, converging-diverging position for obtaining generallyoptimum channeling of the high speed exhaust gases there through duringwet operation. However, when an aircraft using such a conventionalvariable area exhaust nozzle lands and taxis and operates in a groundidle mode, the exhaust nozzle thereof is also positioned in theaugmented mode even though the augmenter is not activated. This is doneto provide a maximum discharge flow area from the exhaust nozzle atground idle operating condition for reducing thrust from the engine.However, since the exhaust nozzle in its augmented mode is designed forthe aerodynamic conditions occurring with an activated augmenter, whenit is operated in such an augmented mode position at ground idle, it isbeing operated off-design. As a result, the thrust from the engineoccurring during the ground idle mode, is relatively low when comparedwith flight operation of the engine, but is relatively high for groundpropulsion of the aircraft and has a substantial absolute value.

In accordance with the preferred embodiment of the present invention, amethod of spoiling, or reducing, thrust from the combustion gases 18discharged from the exhaust nozzle 58 at the ground idle operatingcondition is provided. The method includes the step of positioning thesecondary exhaust flaps 64, as illustrated schematically in FIG. 4, toform a diffuser 68 for the combustion gases 18 at the ground idleoperating condition, with the diffuser being effective for maintainingattachment of the combustion gases 18, without flow separation, alongthe secondary exhaust flap 64. By diffusing the combustion gases 18 inthe diffuser 68, the velocity thereof, and thusly, the thrust thereof,is reduced. In order to obtain acceptable diffusion without flowseparation, the secondary exhaust flaps 64 must be positioned relativeto the longitudinal centerline axis 24, and the combustion gases 18flowing generally parallel thereto, at relatively shallow angles. Theangle of the secondary exhaust flap 64 relative to the longitudinalcenterline axis 24 is designated as half-angle H and preferably has avalue up to about 15° for obtaining diffusion without flow separation.

A maximum amount of diffusion in a minimum amount of longitudinal extentis desired for keeping the exhaust nozzle 58 relatively short andthereby reducing its weight. However, if the half-angle H is too large,undesirable flow separation will occur with an attendant undesirableincrease in thrust from the combustion gases 18 discharged from theexhaust nozzle 58. In order to ensure that separation of the combustiongases 18 does not occur, a preselected flow separation margin is desiredwhich may be predetermined for particular design applications as desiredand indicates the relative ability of the exhaust nozzle 58 to avoidflow separation. For example, an appropriate non-dimensional parameterof flow separation margin could have a value of 100% if the half-angle Hwere 0°, and would have a value of 0 if the half-angle H were selectedto be that angle at which flow separation occurs. In order to obtainacceptable flow separation margin for the preferred embodiment of theinvention, a half-angle H for the secondary exhaust flaps 64 of about10° is preferred.

In the preferred embodiment of the invention, the method of spoilingthrust may further include the step of positioning the primary exhaustflaps 62 to a fully open position at the ground idle operating conditionas also illustrated in FIG. 4. The position of the primary exhaust flaps62 may be defined by the angle A which represents the inclination angleof the primary exhaust flaps 62 relative to the longitudinal centerlineaxis 24. In the preferred embodiment, the inclination angle A isrelatively small and is preferably 0° at the fully open position of theprimary exhaust flaps 62.

Defined at the junction of the primary and secondary flaps 62 and 64 isa conventional throat 70 of the exhaust nozzle 58 which has a relativelyminimum flow area, conventionally denoted as A₈. With the primaryexhaust flap 62 in a fully open position, the throat area A₈ is maximum.Since the secondary exhaust flaps 64 are connected to the primaryexhaust flaps 62 they end at a conventional exhaust outlet 72 having aflow area conventionally denoted A₉. By positioning the primary exhaustflap 62 at a fully open position, and by positioning the secondary flap64 for obtaining diffusion, the exhaust nozzle 58 provides a relativelylarge areas A₈ and A₉ for channeling the combustion gases 18 whilespoiling thrust therefrom during the ground idle mode. With theinclination angle A being preferably 0 in this mode, the primary exhaustflap 62 is positioned generally parallel to the longitudinal centerlineaxis 54.

Unlike a conventional variable exhaust nozzle wherein the position ofthe primary and secondary exhaust flaps is typically the same duringboth a ground idle operating condition and an augmented operatingconditon, an additional feature of the present invention is that theprimary and secondary exhaust flaps 62 and 64 are disposed together indifferent positions during the ground idle operating condition andduring the augmented operating condition, as well as during the dryoperating condition of the engine 12.

More specifically, FIG. 4 illustrates the exhaust nozzle 58 during theground idle operating condition to form the diffuser 68 as a divergingchannel defined by and between the secondary exhaust flaps 64. Theprimary exhaust flaps 62 are positioned generally parallel to thelongitudinal centerline 64 and define therebetween a generally constantarea flow channel 74. In alternate embodiments of the invention, theflow channel 74 may be slightly converging with inclination angles A upto about 5° without significantly adversely affecting the ability of thesecondary exhaust flaps 64 to spoil thrust.

Illustrated in FIG. 5, is the exhaust nozzle 58 positioned during thedry operating condition or mode of the engine 12 to form generally fullyclosed converging and diverging channels 74 and 68, defined between theprimary and secondary exhaust flaps 62 and 64, respectively, forpowering the aircraft 10 in flight at intermediate levels of thrustgreater than the thrust at the ground idle operating condition. Theconverging flow channel 74 formed by the primary flaps 62 during the drymode, is considered generally fully closed since the flow area A₈ at thethroat 70 has relatively minimum values as compared to the other modesof operation of the engine 12. The inclination angle A of the primaryflap 62 is about 35° during such dry operation, and of course may varyduring such dry operation. The diverging channel 68 is also consideredgenerally closed since the outlet area A₉ is also at a relativelyminimum value with the half-angle H having values approaching about 7° .

Illustrated in FIG. 6 is an exemplary intermediate augmented, or wetoperating condition or mode of the engine 12 wherein the primary andsecondary exhaust flaps 62 and 64 are positioned to form generally openconverging and diverging channels 74 and 68, respectively, for poweringthe aircraft 10 in flight at high levels of thrust greater than theintermediate levels of thrust associated with the dry mode. Theconverging channel 74 is considered generally open since the inclinationangle A is relatively small, which occurs when the primary flaps 62 arepositioned for obtaining maximum values of the throat flow area A₈. Thesecondary exhaust flaps 64 are positioned generally open for obtainingrelatively high outlet flow areas A₉, with the half-angle H being, forexample, 15° for the intermediate mode illustrated in FIG. 6.

FIG. 7 is similar to FIG. 6, however it illustrates the position of theprimary and secondary exhaust flaps 62 and 64 during maximum augmentedoperating condition with maximum thrust obtained from the exhaust gases18. The primary and secondary flaps 62 and 64 are considered fully openwith the throat flow area A₈ being maximum and the outlet flow area A₉being maximum. The half-angle H associated with the secondary exhaustflap 64 has a value of about 20° in this exemplary embodiment.

In a conventional aircraft, the exhaust nozzle 58 would be positionedfully open as illustrated in FIG. 7 both during wet operation as well asduring ground idle operation, with an attendant flow separation of theexhaust gases 18 during the ground idle operation and relatively highand undesirable thrust from the engines 12. As described above, bypositioning the primary and secondary flaps 62 and 64 as illustrated inFIG. 4 during the ground idle mode, thrust is spoiled, or reduced. Thisreduces the amount of wheel braking required to prevent aircraft rollingand therefore reduces wear on the brakes. The brakes may be used lightlyor they need not be utilized at all in some embodiments for preventingrolling of the aircraft 10 which would otherwise occur with relativelyhigh ground idle thrust. Safer performance of the aircraft 10 is alsoobtained, especially during icy taxiway and runway conditions, since theundesirable relatively high ground idle thrust is spoiled.

In order to obtain the generally three different positions of theprimary and secondary exhaust flaps 62 and 64 during ground idle, dry,and augmented operating conditions, it is preferred that the secondaryexhaust flaps 64 are positionable in part independently of the primaryexhaust flaps 62. A conventional variable area exhaust nozzle includesprimary and secondary flaps and a fairing disposed in a conventionalfour-bar arrangement with actuators for positioning the flaps. Since theflaps and fairings are arranged in a four-bar arrangement, theirmovements are interdependent, and without additional means would beunable to be positioned in the three required positions during groundidle, dry, and augmented operations described above.

A preferred and exemplary embodiment of an apparatus for carrying outthe method of the present invention is illustrated in FIGS. 8 and 9. Theexhaust nozzle 58, in this axisymmetric embodiment, includes the annularcasing or tailpipe 54, the primary flaps 62, the secondary flaps 64, andthe fairings 66. Each of the primary flaps 62 includes an upstream end76 conventionally pivotally connected to the casing 54, a downstream end78, an inner surface 80 which faces and confines the combustion gases18, and an outer surface 82 formed in part by a stiffening rib extendingbetween the upstream and downstream ends 76 and 78. Each of thesecondary flaps 64 includes an upstream end 84 conventionally pivotallyconnected to the primary exhaust flap downstream end 78, a downstreamend 86, an inner surface 88 facing towards and confining the combustiongases 18, and an outer surface 90 formed in part by a conventionalstifening rib extending between the upstream and downstream ends 84 and86. Conventional seals 92, some of which are shown in FIG. 3 aresuitably positioned between the primary and secondary flaps 62 and 64for sealing exhaust gases from flowing between adjacent primary andsecondary flaps 62 and 64.

Each of the fairings 66 includes an upstream end 94 pivotally connectedto the casing 54 as further described hereinbelow, and a downstream end96 pivotally connected to the secondary exhaust flap downstream end 86.

The exhaust nozzle 58 further includes means 98 for selectivelypositioning the primary and secondary exhaust flaps 62 and 64 during thethree modes of operation: ground idle operating condition, dry operatingcondition, and augmented operating condition as described above. Thepositioning means 98 includes a primary ring 100 surrounding the primaryflaps 62, a plurality of primary actuators 102, such as hydraulicactuators, conventionally operatively connected to the primary ring 100for translating the primary ring 100 parallel to the longitudinalcenterline axis 24. Each of the primary actuators 102 includes anupstream end 104 suitably pivotally connected to the casing 54, by aspherical joint for example, and an extendable rod 106 suitablypivotally connected to the primary ring 100, by a spherical joint, forexample. A plurality of circumferentially spaced primary links 108pivotally join the primary ring 100 to the downstream ends 78 of theprimary flaps 62, for example, by spherical joints.

The positioning means 98 further includes a secondary ring 110 disposedradially outwardly of the primary ring 100 and pivotally joined to thefairing upstream ends 94, for example, by spherical joints. A pluralityof secondary actuators 112, which may be conventional hydraulicactuators, are operatively connected to the secondary ring 110 fortranslating the secondary ring parallel to the longitudinal centerlineaxis 24. Each of the secondary actuators 112 includes an upstream end114 suitably pivotally connected to the casing 54 by a spherical jointfor example, and an extendable rod 116 pivotally connected to thesecondary ring 110 by a spherical joint for example.

During operation, the primary and secondary exhaust flaps 62 and 64 arepositionable by the primary and secondary actuators 102 and 112. Theprimary actuators 102 are effective for translating the primary ring 100which in turn causes the links 108 to rotate the primary flaps 62 aboutthe primary flap upstream ends 76. The primary flaps 62 may thusly berotated and positioned in any of the positions illustrated in FIGS. 4-7and at positions therebetween. The inclination angle A, in the preferredembodiment, may range from about 0° during the ground idle modeillustrated in FIG. 4 to about 35° in the dry mode illustrated in FIG. 5where the primary flaps 62 are fully closed.

As the primary flaps 62 are rotated, the upstream ends 84 of thesecondary exhaust flaps 64 are moved with the movement of the primaryflap downstream ends 78. The secondary flap downstream ends 86 arepositioned by movement of the fairing 66 caused by movement of thesecondary ring 110. The secondary actuators 112 are effective fortranslating the secondary ring 110 which causes the secondary flaps 64to rotate relative to the primary flap downstream ends 78. Accordingly,the secondary flaps 64 are positionable in part independently of theprimary exhaust flaps 62 since the secondary flap downstream ends 86 maybe independently positioned by the secondary ring 110 whereas thesecondary flap upstream ends 84 are positioned with, and therefore aredependent upon the position, of the primary flaps 62. The secondaryflaps 64 are thus positionable by the positioning means 98 in all of thepositions illustrated in FIGS. 4-7.

While there have been described herein what are considered to bepreferred embodiments of the present invention, other modifications ofthe invention shall be apparent to those skilled in the art from theteachings herein, and it is, therefore, desired to be secured in theappended claims, all such modifications as fall within the true spiritand scope of the invention.

More specifically, and for example only, although the invention has beendescribed with respect to an axisymmetric, variable area exhaust nozzle,it may also be utilized with two dimensional converging-diverging typesof exhaust nozzles which are generally rectangular in flow crosssection. It may also be utilized in conjunction with yet other types ofexhaust nozzles, including non-symmetric nozzles.

Furthermore, although the positioning means 98 as disclosed above ispreferred, other means for positioning the primary and secondary exhaustflaps 62 and 64 for obtaining all of the positions thereof asillustrated in FIGS. 4-7 may be utilized in accordance with theinvention for obtaining thrust spoiling at ground idle operatingcondition, while also being able to position the primary and secondaryflaps 62 and 64 at different positions for both dry and augmentedoperations.

Accordingly, what is desired to be secured by Letters Patent of theUnited States is the invention as defined and differentiated in thefollowing claims:

I claim:
 1. An exhaust nozzle for an aircraft engine comprising:acasing; a primary exhaust flap having an upstream end pivotallyconnected to said casing, a downstream end, and an inner surface forchanneling combustion gases; a secondary exhaust flap having an upstreamend pivotally connected to said primary exhaust flap downstream end, adownstream end, and an inner surface for channeling combustion gases; afairing having an upstream end pivotally connected to said casing and adownstream end pivotally connected to said secondary exhaust flapdownstream end; and means for selectively positioning said primary andsecondary exhaust flaps together in different positions:during a groundidle operating condition to position said primary flap to a fully openposition, and to form a diverging channel defined by said secondaryexhaust flap for diffusing said combustion gases while maintainingattachment of said combustion gases along said secondary exhaust flapfor spoiling thrust; during a dry operating condition to form generallyclosed converging and diverging channels defined by said primary andsecondary exhaust flaps, respectively, for powering an aircraft inflight at intermediate levels of thrust greater than said spoiled thrustat said ground idle operating condition; and during a maximum augmentedoperating condition to form fully open converging and diverging channelsdefined by said primary and secondary exhaust flaps, respectively, forpowering said aircraft in flight at maximum thrust greater than saidintermediate levels of thrust.
 2. An exhaust nozzle according to claim 1wherein said positioning means is effective for positioning said primaryexhaust flap to a fully open position generally parallel to alongitudinal centerline axis of said exhaust nozzle at said ground idleoperating condition.
 3. An exhaust nozzle according to claim 1 whereinsaid positioning means is effective for positioning said secondaryexhaust flap downstream end independently of said primary exhaust flap.4. An exhaust nozzle according to claim 3 further including:a pluralityof circumferentially spaced ones of said primary exhaust flaps disposedaxisymmetrically about a longitudinal centerline axis of said nozzle; aplurality of circumferentially spaced ones of said secondary exhaustflaps; and a plurality of circumferentially spaced ones of saidfairings; and wherein said positioning means comprises:a primary ringsurrounding said primary exhaust flaps; a plurality of primary actuatorsoperatively connected to said primary ring for translating said primaryring parallel to said longitudinal centerline axis; a plurality ofcircumferentially spaced primary links pivotally joining said primaryring to said primary exhaust flaps; a secondary ring pivotally joined tosaid fairing upstream ends; and a plurality of secondary actuatorsoperatively connected to said secondary ring for translating saidsecondary ring parallel to said longitudinal centerline axis.
 5. Anexhaust nozzle according to claim 4 wherein said positioning means iseffective for positioning said primary exhaust flaps to a fully openposition generally parallel to said longitudinal centerline axis of saidexhaust nozzle, and positioning said secondary exhaust flaps at anglesrelative to said longitudinal centerline axis up to about 15° at saidground idle operating condition.
 6. An exhaust nozzle according to claim5 wherein said positioning means is effective for positioning saidsecondary exhaust flaps at an angle relative to said longitudinalcenterline axis of about 10° for maintaining a preselected flowseparation margin of said combustion gases flowable along said secondaryexhaust flap inner surfaces.
 7. An exhaust nozzle according to claim 4in combination with said gas turbine engine, said engine including acompressor effective for providing bleed-air at a pressure of at least40 psia at said ground idle operating condition, and said positioningmeans is effective for positioning said primary exhaust flaps to a fullyopen position generally parallel to said longitudinal centerline axis ofsaid exhaust nozzle, and positioning said secondary exhaust flaps atangles relative to said longitudinal centerline axis up to about 15° atsaid ground idle operating condition.
 8. An exhaust nozzle according toclaim 4 in combination with said gas turbine engine in an aircrafthaving wheels and said positioning means is effective for positioningsaid primary exhaust flaps to a fully open position generally parallelto said longitudinal centerline axis of said exhaust nozzle, andpositioning said secondary exhaust flaps at said ground idle conditionat angles relative to said longitudinal centerline axis up to about 15°so that thrust from said engine is insufficient to cause said aircraftto roll on said wheels.